1. _Rhizopods:_--Body uniform in consistence. Pseudopods protruded from any point. 2. _Protoplasta:_--Outer protoplasm slightly firmer in consistence. Pseudopods protruded from any point. (Often grouped with the _Rhizopods_.) 3. _Radiolaria:_--Possessing a central membranous capsule. Usually supported by a flinty skeleton. 4. _Infusoria:_--Outer protoplasm firmer and denser; therefore of more definite shape. Possess permanent threadlike extensions of protoplasm instead of pseudopods. We shall now observe the principal marine members of the protozoa, commencing with the lowest forms, and dealing with each in its proper zoological order as expressed in the above table. MARINE RHIZOPODS When we stand on a beach of fine sand on a very calm day watching the progress of the ripples over the sand as the tide recedes we frequently observe whitish lines marking the limits reached by the successive ripples as they advance toward the shore. If, now, we scrape up a little of the surface sand, following the exact course of one of these whitish streaks, and examine the material obtained by the aid of a good lens, we shall in all probability discover a number of minute shells among the grains of sand. These shells are of various shapes--little spheres, discs, rods, spirals, &c.; but all resemble each other in that they are perforated with a number of minute holes or _foramina_. They are the skeletons of protozoons, belonging to the class _Rhizopoda_, and they exist in enormous quantities on the beds of certain seas. [Illustration: FIG. 54.--A GROUP OF FORAMINIFERS, MAGNIFIED] We will first examine the shells, and then study the nature of the little animals that inhabit them. The shells vary very much in general appearance as well as in shape. Some are of an opaque, dead white, the surface somewhat resembling that of a piece of unglazed porcelain; others more nearly resemble glazed porcelain, while some present quite a vitreous appearance, much after the nature of opal. In all cases, however, the material is the same, all the shells consisting of carbonate of lime, having thus the same chemical composition as chalk, limestones, and marble. If hydrochloric acid be added to some of these shells, they are immediately attacked by the acid and are dissolved in a very short time, the solution being accompanied by an effervescence due to the escape of carbonic acid gas. The shells vary in size from about one-twelfth to one three-hundredth of an inch, and consist either of a single chamber, or of many chambers separated from each other by perforated partitions of the same material. Sometimes these chambers are arranged in a straight line, but more frequently in the form of a single or double spiral. In some cases, however, the arrangement of chambers is very complex.